In the past, the PS plate comprising a hydrophilic support and provided thereon, an oleophilic resin layer was widely utilized as a planographic printing plate material (planographic printing plate original plate), and a desired printing plate was produced usually by dissolving and removing non-image portions after conducting a mask lithography process via a lith film as a plate making process.
Presently, digital technique electronically processing, storing and outputting image information employing a computer has widely spread, and a variety of newly developed image outputting systems corresponding to such the digital technique have been put to practical use, whereby a computer-to-plate (CTP) system with which a printing plate is directly prepared via no lith film by scanning highly directional light such as laser light according to digitalized image information is demanded, and development of a photopolymerizable composition which is curable in high sensitivity and adaptable for this has actively done.
Among planographic printing plate materials suitable for such the CTP system, a planographic printing plate material comprising a polymerization type photosensitive layer containing a polymerizable compound and a binder polymer is utilized in the field of printing in which high printing durability is demanded. As the polymerization type photosensitive layer, commonly known is a photosensitive layer containing an alkali-soluble (meth)acrylic copolymer, a polymerizable (meth)acrylic acid ester and a polymerization intiator (refer to Patent Document 1, for example).
Further, as to the planographic printing plate material containing these polymerization type photosensitive layers, the photosensitive layer suffers from the effects of oxygen in the air, even though the photosensitive layer is formed, whereby reduced sensitivity and insufficient storage stability result. As a method of suffering from less effects of oxygen in the air, known is a method of providing an oxygen-shielding layer (protective layer) on a photosensitive layer (refer to Patent Documents 2 and 3, for example) .
In order to improve printing durability desired for the planographic printing plate material, higher glass transition point (Tg) of a binder polymer to enhance strength of a photosensitive layer is effective, but there are still problems such that solubility to an alkali developer and adhesion to an oxygen-shielding layer are deteriorated. Further, exposure at high-humidity causes a problem such that sensitivity is lowered, and dot reproduction is deteriorated since oxygen permeability is high.
(Patent Document 1) Japanese Patent O.P.I. Publication No. 2003-295426
(Patent Document 2) Japanese Patent O.P.I. Publication No. 2002-169272
(Patent Document 3) Japanese Patent O.P.I. Publication No. 9-204049